It has been found that aerodynamic compensation of ink drops in an ink jet printer for interactive effects alone is not enough for high quality printing. Typical drop interactive effects are charge repulsion between drops, charge induction between drops and aerodynamic drag. However, environmental effects independent of the drop interactions can also cause printing errors. These independent effects include such things as air pressure, air temperature, humidity, nozzle diameter, ink density, flight distance, angle of impact, charging channel width, and other physical characteristics of the environment of the ink stream.
There are numerous techniques for compensating an ink jet printer for drop interactive effects. U.S. Pat. Nos. 3,828,354, 3,946,399 and commonly assigned patent application Ser. No. 23,813, filed Mar. 26, 1979, all teach compensating for the error effects due to drop interactive effects. The Zaretsky U.S. Pat. No. 3,946,399 teaches monitoring the data pattern of an ink jet stream to detect particular print data patterns. These print data patterns are then logically analyzed to select a compensation charge signal to be applied to the charge electrode in the ink jet printer.
The Hilton U.S. Pat. No. 3,823,354 teaches monitoring a seven bit print data pattern to generate the compensation signal for aerodynamic and charge induced effects. Hilton monitors four drops ahead of the reference drop and two drops behind the reference drop and the reference drop itself. Based upon the binary pattern for these seven drops, Hilton addresses a read-only-store memory which contains predetermined compensation values for each possible address.
Patent application Ser. No. 23,813, monitors a large number of drops in the print data pattern to make more accurate compensation decisions. To keep the data processing manageable, only the closest drops to the drop being charged are monitored individually. The more remote drops are monitored as one or more groups of drops contributing a group effect to be compensated for.
None of the above patented techniques deal with the problem of compensating for environmental aerodynamic effects independent of drop interactions. In fact, all of the above techniques can be improved by use of the present invention to compensate environmental effects independent of the print data pattern.
It is known to compensate an ink jet printer for effects which are independent of the print data pattern. However, this has been done in a stationary print media environment where aerodynamic effects were of no consequence to the printing operation. An example is U.S. Pat. No. 3,971,039 issued to Takano et al. The Takano patent teaches varying the charge voltage or the deflection voltage used in a raster type ink jet printer in accordance with ink temperature. However, the patent is aimed at solving a different problem, height control of the raster pattern. Since the print medium does not move in the raster direction during printing of each raster scan, there are no aerodynamic effects causing drop flight time variations and thus print errors. The patent is only concerned about flight direction and not flight time of the drops.
If the print medium does move, an ink jet printer must correct for drop velocity variations and aerodynamic effects on the drops because flight time is a factor in print error. The present invention involves this type of printer, a printer where flight time is a factor. Drop velocity in such a printer is usually controlled by a drop velocity servo that controls ink pressure at the nozzle. These servos are well known and form no part of the present invention. The present invention is directed to solving the problem of print errors caused by aerodynamic effects where those effects are due to environmental effects as well as drop interactive effects.